The present invention relates to an electronic still camera and its signal processing method and, more particularly, to an electronic still camera which removes dark output components resulting from an image sensing element such as a CCD (Charge Coupled Device) or the like, and its signal processing method.
In recent years, electronic still cameras have rapidly prevailed. Such electronic still camera generates a still image of an object using an image sensing element such as a CCD or the like, and electronically records the generated image in a storage medium such as a semiconductor memory or the like.
An image sensing element such as a CCD or the like which is used in such electronic still camera has so-called dark output (dark voltage) characteristics; it produces an output even when incoming light is shielded, thus producing a very weak current called a dark current.
The dark voltage in the CCD becomes a noise component, and consequently adversely influences image data, thus deteriorating image quality.
As one method of solving this problem, when image sensing is instructed by shutter operation, it is done twice at an identical shutter speed (identical exposure time) with a mechanical shutter opened and closed and, after that, second image data (image data consisting of only dark output components) obtained by image sensing with the mechanical shutter closed is subtracted from first image data (object image data containing dark output components) obtained by image sensing with the mechanical shutter opened, thus removing the noise components from the first image data as sensed image data.
With this method, image quality can be prevented from deteriorating, and high-quality image data can be obtained.
Since the dark voltage (dark output components) in the CCD has temperature dependence (the dark voltage roughly doubles upon a change (increase) in ambient temperature of around 8° C.), and exposure time (charge accumulation period) dependence (the dark current tends to increase as the exposure time becomes longer), two image sensing operations are done at substantially the same timings (the same temperature) and the same exposure time to equalize the levels of the dark output components contained in the first image data and the second image data (dark output components).
However, such method can prevent image quality from deteriorating and can obtain high-quality image data, but requires two image sensing operations upon instruction of image sensing. For this reason, the time required for image sensing doubles if no other conditions are considered, and disturbs continuous image sensing.
This problem becomes more serious when image sensing must be done in a dark place, since a long exposure time must be assured.
It is an object of the present invention to provide an electronic still camera which can obtain high-quality image data while preventing deterioration of image quality, and can shorten the time required for image sensing, and its signal processing method.